Hello world example by using an headerless implementation of the WASI interface version 0.1 . The only dependency is clang.

hello.c

/*
 * This code is a headerless implementation of the WASI interface version 0.1 in C and prints "Hello World!. 
 * You only need clang.
 *
 * compile with
 *     clang -Os -nostdlib --target=wasm32 hello.c -o hello.wasm
 *
 * run with
 *    https://runno.dev/wasi
 *        Just upload hello.wasm
 *
 *    https://github.com/wasmerio/wasmer
 *        wasmer run hello.wasm
 *
 *    https://github.com/CraneStation/wasmtime
 *        wasmtime hello.wasm
 *
 *    https://github.com/fastly/lucet/wiki/lucetc
 *        lucetc-wasi hello.wasm -o hello.so && lucet-wasi hello.so
 */

// WASI FD_WRITE FUNCTION DECLARATIONS
// ----------------------------------------------------
// see https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/headers/public/wasi/api.h

// Custom data type definitions
typedef unsigned short uint16_t;
typedef unsigned int uint32_t;
typedef long size_t;
typedef uint16_t __wasi_errno_t;
typedef uint32_t __wasi_fd_t;

// Structure to hold the buffer and its length for WASI I/O operations
typedef struct __wasi_ciovec_t {
    const void *buf;
    size_t buf_len;
} __wasi_ciovec_t;

// Macro to define syscall name for WASI functions
#define __WASI_SYSCALL_NAME(name) \
    __attribute__((__import_module__("wasi_snapshot_preview1"), __import_name__(#name)))

// Function prototype for the WASI fd_write function
__wasi_errno_t __wasi_fd_write(
    __wasi_fd_t fd,
    const __wasi_ciovec_t *iovs,
    size_t iovs_len,
    size_t *nwritten
) __WASI_SYSCALL_NAME(fd_write);

// UTILITY FUNCTIONS
// ----------------------------------------------------

// Function to return the length of a string
size_t slen(const char *str)
{
    const char *char_ptr;
    for (char_ptr = str; *char_ptr; ++char_ptr)
        ;
    return (char_ptr - str);
}

// Function to print a string to stdout using the WASI fd_write function
void print(const char *str)
{
    const __wasi_fd_t stdout = 1; // File descriptor for stdout
    size_t nwritten; // Number of bytes written
    __wasi_errno_t error; // Error code returned by WASI function
    __wasi_ciovec_t iovec; // I/O vector for WASI write operation

    iovec.buf = str;
    iovec.buf_len = slen(str);
    error =__wasi_fd_write(stdout, &iovec, 1, &nwritten);
}

// MAIN ENTRY POINT
// ----------------------------------------------------
__attribute__((export_name("_start")))
void _start()
{
    print("Hello World!\n");  // Print "Hello World!" to stdout
}